The public is being asked to make a choice: an airport with or without a third runway by 2030. But the government has said little so far on the social and environmental costs of the project.

The third runway - with a cost of HK$136.2 billion, factoring in inflation - would be the city's most expensive single project, but the Airport Authority said yesterday it would bring eight times as much in economic benefits over the next 50 years. The cost was put at HK$86.2 billion in 2010 dollar terms.

If the plan is rejected, the authority says, the airport's two runways will run out of capacity by 2020. Hong Kong will stand to lose 23 million air passengers, 2.9 million tonnes of cargo volume, 182,000 aircraft movements and 96,000 jobs that the city could have had in 2030.

'We have this world-class airport now because our predecessors planned it for us,' said Transport and Housing Secretary Eva Cheng, as authorities unveiled a long-awaited master plan for airport development. 'We should plan ahead now if we want to keep our edge.'

While the project has the support of the aviation and tourism industries, economists, professionals and pro-administration lawmakers, it may be a tougher sell with the public.

It requires reclamation of 650 hectares - half the present size of the airport. It would be the second largest reclamation in local history. Executives said a new reclamation method would be used and a new flight path drawn to minimise noise, but a solid compensation plan will not be produced until they apply for an environmental permit.

Officials yesterday gave no details on how the government would bridge a funding gap expected to reach HK$112.8 billion after netting the authority's contribution.

'We will talk about that in our next step. Of course, the less taxpayers' money required, the better,' Cheng said. 'The airport authority can apply for a loan or issue bonds.'

The authority may even review an old proposal to raise funds via a listing on the stock exchange. But all these will only be discussed after a decision is made on the third runway.

The authority will submit its recommendation to the government later this year after consultation ends in September, but it is unlikely the administration will reach a decision before the new chief executive is elected in the latter part of next year.

Supporters of the project want construction to start by 2013 or 2014, but transport analyst and environmentalist Hung Wing-tat said the public should be given more time to ponder the consequences.

'The economy will grow continuously and we can't always be struggling to catch up,' Hung said. 'Where does it end? It isn't like there are no real and social costs to pay.'

Some have queried whether better co-operation between the Pearl River Delta's five airports could reduce the need for the runway. But an authority spokesman said directing flights elsewhere was unfeasible due to air service agreement constraints.